1. Field of the Invention
This disclosure generally relates to automatic data collection (ADC) devices, for example, machine-readable symbol readers such as barcode scanners.
2. Description of the Related Art
Readers for reading machine-readable symbols are generally categorized into two groups: 1) moving beam devices commonly referred to as scanners (e.g., laser scanners), and 2) fixed beam devices commonly referred to as imagers (e.g., CCD imagers). Each group has its own relative advantages.
Moving beam devices typically move or scan a light across a target. For example, a scanner may employ a laser diode and a mechanism for moving a laser beam produced by the laser diode across the target. While it may be possible to move the light source itself, scanners typically employ one or more rotating prisms or oscillating mirrors that reflect the laser beam, sweeping the laser beam back and forth across a target, and thereby sequentially illuminating portions of the target along a scan line. Scanners also typically include an optoelectronic sensor or light detector, for example one or more photodiodes. The optoelectronic sensor detects the laser light returned (e.g., reflected) from the target, and produces a corresponding analog signal. The scanner may employ a standard lens or retro-collector to focus the returned light on the optoelectronic sensor. Typically, the scanner, or an associated device, converts an analog signal produced by the optoelectronic sensor into to a digital signal, before decoding the digital signal according to standard decoding schemes.
While fixed beam devices may rely on ambient light, most imagers employ a flood illumination subsystem. The flood illumination subsystem typically includes a number of high intensity light emitting diodes (LEDs) arranged to simultaneously flood the entire target with light. Imagers strive for uniform illumination over the entire target. Imagers also include an optoelectronic sensor, e.g., one- or two-dimensional arrays of charge coupled devices (CODs), and may include a lens assembly to focus returned light onto the optoelectronic sensor. A CCD array may be electronically sampled or scanned, to produce a digital signal suitable for decoding.
Typically, scanners include non-imaging optics that do not rely upon the quality of the image, but instead focus on the transfer efficiency (i.e., the ratio of the energy directed to the detector over the energy entering the collector). For example, U.S. Pat. No. 5,357,101 describes a barcode scanner using a non-imaging compound parabolic collector. While a compound parabolic collector is efficient at rejecting unwanted light, there is no simple way to change the field of view. Furthermore, an ideal compound parabolic collectors, or line flow concentrators, is very long. Thus, compound parabolic collectors are often truncated or used with a first imaging stage at the expense of transfer efficiency and cost. The use of compound parabolic collectors in Micro-Electronic-Mechanical Systems (MEMS) devices is severely limited by their relatively large size.
There is a need for a small light collector that can be used with very small illumination subsystems, such as MEMS based illumination subsystems. There is also need for a light collector with the ability to reject unwanted light. Hence there is a need in automatic data collection arts for an optoelectronic reader that can overcome at least some of the aforementioned drawbacks.